Re: [PATCH v30 2/3] mailbox: pcc: functions for reading and writing PCC extended data

[Sudeep Holla <sudeep.holla@arm.com>]

On Thu, Oct 16, 2025 at 05:02:20PM -0400, Adam Young wrote:
> Adds functions that aid in compliance with the PCC protocol by
> checking the command complete flag status.
> 
> Adds a function that exposes the size of the shared buffer without
> activating the channel.
> 
> Adds a function that allows a client to query the number of bytes
> avaialbel to read in order to preallocate buffers for reading.
> 
> Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>
> ---
>  drivers/mailbox/pcc.c | 129 ++++++++++++++++++++++++++++++++++++++++++
>  include/acpi/pcc.h    |  38 +++++++++++++
>  2 files changed, 167 insertions(+)
> 
> diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
> index 978a7b674946..653897d61db5 100644
> --- a/drivers/mailbox/pcc.c
> +++ b/drivers/mailbox/pcc.c
> @@ -367,6 +367,46 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>  	return IRQ_HANDLED;
>  }
>  
> +static
> +struct pcc_chan_info *lookup_channel_info(int subspace_id)
> +{
> +	struct pcc_chan_info *pchan;
> +	struct mbox_chan *chan;
> +
> +	if (subspace_id < 0 || subspace_id >= pcc_chan_count)
> +		return ERR_PTR(-ENOENT);
> +
> +	pchan = chan_info + subspace_id;
> +	chan = pchan->chan.mchan;
> +	if (IS_ERR(chan) || chan->cl) {
> +		pr_err("Channel not found for idx: %d\n", subspace_id);
> +		return ERR_PTR(-EBUSY);
> +	}
> +	return pchan;
> +}
> +
> +/**
> + * pcc_mbox_buffer_size - PCC clients call this function to
> + *		request the size of the shared buffer in cases
> + *              where requesting the channel would prematurely
> + *              trigger channel activation and message delivery.
> + * @subspace_id: The PCC Subspace index as parsed in the PCC client
> + *		ACPI package. This is used to lookup the array of PCC
> + *		subspaces as parsed by the PCC Mailbox controller.
> + *
> + * Return: The size of the shared buffer.
> + */
> +int pcc_mbox_buffer_size(int index)
> +{
> +	struct pcc_chan_info *pchan = lookup_channel_info(index);
> +
> +	if (IS_ERR(pchan))
> +		return -1;
> +	return pchan->chan.shmem_size;
> +}
> +EXPORT_SYMBOL_GPL(pcc_mbox_buffer_size);
> +

Why do you need to export this when you can grab this from
struct pcc_mbox_chan which is returned from pcc_mbox_request_channel().

Please drop the above 2 functions completely.

> +
>  /**
>   * pcc_mbox_request_channel - PCC clients call this function to
>   *		request a pointer to their PCC subspace, from which they
> @@ -437,6 +477,95 @@ void pcc_mbox_free_channel(struct pcc_mbox_chan *pchan)
>  }
>  EXPORT_SYMBOL_GPL(pcc_mbox_free_channel);
>  
> +/**
> + * pcc_mbox_query_bytes_available
> + *
> + * @pchan pointer to channel associated with buffer
> + * Return: the number of bytes available to read from the shared buffer
> + */
> +int pcc_mbox_query_bytes_available(struct pcc_mbox_chan *pchan)
> +{
> +	struct pcc_extended_header pcc_header;
> +	struct pcc_chan_info *pinfo = pchan->mchan->con_priv;
> +	int data_len;
> +	u64 val;
> +
> +	pcc_chan_reg_read(&pinfo->cmd_complete, &val);
> +	if (val) {
> +		pr_info("%s Buffer not enabled for reading", __func__);
> +		return -1;
> +	}

Why would you call pcc_mbox_query_bytes_available() if the transfer is
not complete ?

> +	memcpy_fromio(&pcc_header, pchan->shmem,
> +		      sizeof(pcc_header));
> +	data_len = pcc_header.length - sizeof(u32) + sizeof(pcc_header);

Why are you adding the header size to the length above ?

> +	return data_len;
> +}
> +EXPORT_SYMBOL_GPL(pcc_mbox_query_bytes_available);
> +
> +/**
> + * pcc_mbox_read_from_buffer - Copy bytes from shared buffer into data
> + *
> + * @pchan - channel associated with the shared buffer
> + * @len - number of bytes to read
> + * @data - pointer to memory in which to write the data from the
> + *         shared buffer
> + *
> + * Return: number of bytes read and written into daa
> + */
> +int pcc_mbox_read_from_buffer(struct pcc_mbox_chan *pchan, int len, void *data)
> +{
> +	struct pcc_chan_info *pinfo = pchan->mchan->con_priv;
> +	int data_len;
> +	u64 val;
> +
> +	pcc_chan_reg_read(&pinfo->cmd_complete, &val);
> +	if (val) {
> +		pr_info("%s buffer not enabled for reading", __func__);
> +		return -1;
> +	}

Ditto as above, why is this check necessary ?

> +	data_len  = pcc_mbox_query_bytes_available(pchan);
> +	if (len < data_len)
> +		data_len = len;
> +	memcpy_fromio(data, pchan->shmem, len);
> +	return len;
> +}
> +EXPORT_SYMBOL_GPL(pcc_mbox_read_from_buffer);
> +
> +/**
> + * pcc_mbox_write_to_buffer, copy the contents of the data
> + * pointer to the shared buffer.  Confirms that the command
> + * flag has been set prior to writing.  Data should be a
> + * properly formatted extended data buffer.
> + * pcc_mbox_write_to_buffer
> + * @pchan: channel
> + * @len: Length of the overall buffer passed in, including the
> + *       Entire header. The length value in the shared buffer header
> + *       Will be calculated from len.
> + * @data: Client specific data to be written to the shared buffer.
> + * Return: number of bytes written to the buffer.
> + */
> +int pcc_mbox_write_to_buffer(struct pcc_mbox_chan *pchan, int len, void *data)
> +{
> +	struct pcc_extended_header *pcc_header = data;
> +	struct mbox_chan *mbox_chan = pchan->mchan;
> +
> +	/*
> +	 * The PCC header length includes the command field
> +	 * but not the other values from the header.
> +	 */
> +	pcc_header->length = len - sizeof(struct pcc_extended_header) + sizeof(u32);
> +
> +	if (!pcc_last_tx_done(mbox_chan)) {
> +		pr_info("%s pchan->cmd_complete not set.", __func__);
> +		return 0;
> +	}

The mailbox moves to next message only if the last tx is done. Why is
this check necessary ?

> +	memcpy_toio(pchan->shmem,  data, len);
> +
> +	return len;
> +}
> +EXPORT_SYMBOL_GPL(pcc_mbox_write_to_buffer);
> +
> 

I am thinking if reading and writing to shmem can be made inline helper.
Let me try to hack up something add see how that would look like.

>  /**
>   * pcc_send_data - Called from Mailbox Controller code. Used
>   *		here only to ring the channel doorbell. The PCC client
> diff --git a/include/acpi/pcc.h b/include/acpi/pcc.h
> index 840bfc95bae3..96a6f85fc1ba 100644
> --- a/include/acpi/pcc.h
> +++ b/include/acpi/pcc.h
> @@ -19,6 +19,13 @@ struct pcc_mbox_chan {
>  	u16 min_turnaround_time;
>  };
>  
> +struct pcc_extended_header {
> +	u32 signature;
> +	u32 flags;
> +	u32 length;
> +	u32 command;
> +};
> +

This again is a duplicate of struct acpi_pcct_ext_pcc_shared_memory.
It can be dropped.

-- 
Regards,
Sudeep